Magneto-optical reading device for multi-track magnetic tapes

ABSTRACT

The disclosure relates to a magneto-optical reading device for multi-track magnetic tapes, including a flat large incident beam directed onto the active part of a magneto-optical read head and reflected by this head onto an array of sensors, characterized in that it includes a correction device able to move the zone of incidence of the beam on the read head in order to keep the beam optimally positioned on the active part of the read head as the latter suffers progressive wear. The invention increases the useful working life of such a reading device.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to devices able to read magneto-opticallyand simultaneously all the parallel tracks recorded on a magnetic tape.It is notably applicable to magnetic tape systems for recordinghigh-density digital data, known under the name SDCR (Static DigitalCassette Recording).

2. Description of the Related Art

The SDCR high-density magnetic tape reading/recording system wasdesigned in the central R&D laboratory of the THOMSON-CSF company withthe goal of producing a high-density magnetic tape recording systemoperating simultaneously on a set of parallel tracks, without usingclassic rotating heads. For this purpose the SDCR uses a highly originalrecording device incorporating a magnetic head having a set ofintegrated poles laid out matrix-fashion, and a Kerr effectmagneto-optical read head of substantially monolithic structure. Thewrite and read heads therefore enable parallel recording and reading ofextremely high data density, despite the fact that the fabrication ofthe SDCR is very simple compared with rival systems.

More particularly, the Kerr effect read head includes, in a manner nowknown to professionals of the art, a prism comprising a stack ofsuitable materials. It is illuminated by a laser producing a flatpolarized incident beam whose thickness is adapted to the dimension ofthe bits written on the magnetic tracks, and whose width is sufficientto cover the whole breadth of the magnetic tape. After reflection on thesensitive layer of the head, the polarization of the optical beamchanges according to the direction of the magnetization induced by eachrecorded track of the magnetic tape. This polarization change istransformed into an intensity change, by a polarizer for example. Thebeam is then received on an array of sensors, of CCD (charge-coupleddevice) type for example. Each of the cells of this CCD then delivers asignal representative of the information recorded on each of the tapetracks.

The system described succinctly above has been divulged in more detailin a set of patents describing the global architecture of the system andcertain special aspects of its fabrication. We can notably mention thefollowing French patents filed by the applicant:

-   8917313 (28 Dec. 1989), publication no. 2 656 723,-   84 07761 (18 May 1984), publication no. 2 564 674,-   93 01407 (7 Feb. 1993), publication no. 2 701 332,-   90 00546 (18 Jan. 1990), publication no. 2 657 100,-   92 11146 (18 Sep. 1992), publication no. 2 696 037,-   8614974 (28 Oct. 1986), publication no. 2 605 783,-   87 14818 (4 May 1987), publication no. 2 622 335,-   88 05592 (27 Apr. 1988), publication no. 2 630 853,-   96 08393 (5 Jul. 1996), publication no. 2 750 787.

This system has enabled satisfactory prototypes to be built. Experimentson these prototypes have however revealed a number of problems whoseresolution would enhance both the performance and working life ofdevices made according to this system.

One of these problems concerns the wear of the magnetic read head.

Although this read head is made from particularly abrasion-resistantmaterials, it is still subject to wear caused by the movement of themagnetic tape over it, which leads to progressive degradation of thehead's performance and ultimately to its failure. This is moreovercommon to all systems in which there is contact between the read headand the magnetic tape, in particular the systems with rotating headsused today.

SUMMARY OF THE INVENTION

To overcome this disadvantage, or at least to significantly slow downthis phenomenon, the invention proposes a magneto-optical reading devicefor multi-track magnetic tapes, including a flat large incident beamdirected onto the active part of a magneto-optical read head andreflected by this head onto an array of sensors, characterized in thatit includes correction means able to move the zone of incidence of saidbeam on said read head in order to keep said beam optimally positionedon said active part of said read head as the latter suffers progressivewear.

According to a characteristic of the invention, said correction meansinclude a first flat parallel-faced plate positioned on the path of saidincident beam and rotatable around a first axis parallel to the plane ofsaid incident beam and perpendicular to its axis.

According to another characteristic, said correction means also includea second flat parallel-faced plate positioned on the path of thereflected beam returned by said read head and rotatable around a secondaxis parallel to the plane of said reflected beam and perpendicular toits axis, this second plate being provided to correct the displacementof the reflected beam associated with the displacement of said incidentbeam caused by the rotation of said first plate.

According to another characteristic of the invention, said second platealso includes a third rotation axis perpendicular to the plane of saidreflected beam so as to be able to move this reflected beam in its ownplane in order to keep the spot beams of this beam modulated by saidmagnetic tape tracks on the corresponding sensors of said detectionarray, and thereby compensate for any wandering movements of themagnetic tape.

According to another characteristic of the invention, the deviceincludes a controlled system that drives said correction means to assurepermanent and optimal compensation for wear of said read head.

Other characteristics and advantages of the invention will become clearon reading the following description of a preferred embodiment, takenonly as non-limitative example, with reference to the attached drawingwhich shows schematically a read head according to the invention inoperation with a magnetic tape.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is an exemplary embodiment of a magneto-optical reading device ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the FIGURE, a magnetic tape 101 carrying a set of parallel tracks 102moves over a read head 103. The structure of this head is known and istherefore shown only schematically and transparently to show the area ofcontact of the head with the magnetic tape.

The active part 104, magnetized by the tape, is a very narrow straightstrip (about 2 to 3 microns wide). In the FIGURE it is shown at theextreme left-hand edge of the read head 103.

This active part is illuminated by a laser (not shown in the FIGURE)whose polarized light beam is shaped by a known optical system (notshown) such that its dimensions match those of the active part 104.

This beam 105, oriented to illuminate the active part 104, is reflectedfrom this active part 104 with its polarization modulated according tothe orientation of the bits in the magnetic tracks 102, to form a returnbeam 106 comprising a series of modulated spot beams which are directedonto an array 107 of sensors, of CCD type for example. Each sensor 108in this array receives one of the spot beams of the beam 106 which hasbeen modulated by one of the tracks 102. The intensity received by eachof these cells depends on the modification of the beam's polarizationcaused by the magnetic state of the track being read. Polarizing plates(not shown, but known to professionals of the art) are used to transformthis polarization change into an intensity change that can be detectedby the cells 108.

The interest of this system is that the reading of the tracks is notindividualized at the actual head 103, unlike magneto-resistive headswhich necessitate a longitudinal segmentation adapted to the tracks anda tracking system to follow the tracks on the magnetic tape andcompensate the lateral wandering of the tape, both of which are verydifficult to implement.

Individualization of the tracks is therefore transferred to the array ofsensors 107, which is a well-known and commercially-available device.

Compensation for tape wandering is achieved using a flat parallel-facedplate 109 that can rotate about an axis 110 perpendicular to the planeof the beam 106. The movement ω_(x) thus obtained enables this beam 106to be moved in its own plane while remaining parallel to itself,according to the well-known optical properties of flat parallel-facedplates. A servo system, of known type, is used to keep the image of thetracks 102 on the cells 108 to which they are assigned, such that eachof these cells always delivers the signal recorded on the correspondingtrack with the maximal level possible—and despite movements of themagnetic tape in its own plane.

To operate correctly the system requires contact of the magnetic tape101 with the head 103. This inevitably leads to wear of the head 103over time, especially in very high bit-rate systems in which the tapemay be spooled at very high speed. Although this wear is very slight inabsolute terms, owing to the very small dimensions of the active part104, its relative value is not negligible. Consequently means ofcompensating for this wear is essential if the device is to have areasonable working lifetime.

In the example shown in the FIGURE, the active part 104 will bedisplaced by a distance Z after a few hundred hours of operation, to aposition 111 which is situated off the plane of the incident beam 105.

To be able to move this incident beam onto the zone 111, the inventionprovides for a second flat parallel-faced plate 112 that can rotateabout an axis 113 parallel to the plane of the incident beam 105 andperpendicular to the beam's axis. In this manner, and according to thewell-known optical action of flat parallel-faced plates, the movementω_(z′) of this flat parallel-faced plate moves the incident beam 105 ina direction perpendicular to its plane beam while maintaining itparallel to itself. This beam can therefore be moved to the new position111 of the active part of the read head.

The return beam 116 reflected by this zone at its new position 111 is ofcourse shifted relative to the initial reflected beam 106 and thereforeno longer lands in the right position on the sensor array 107.

To compensate this shift of the return beam, the invention also proposesto turn the flat parallel-faced plate 109 about an axis 114 parallel tothe plane of the return beam 106 and perpendicular to its axis. Themovement ω_(z″) of the plate 109 around this axis 114 will besubstantially symmetrical to the movement ω_(z′) of the plate 112 aroundthe axis 113 and will compensate the displacement of the return beamassociated with the displacement of the incident beam, such that thisreflected beam always lands at the right position on the sensor array107.

These displacements of the incident and reflected beams, which need tobe made only extremely slowly, can be obtained by micromotors,piezoelectric for example (not shown in the FIGURE) and controlled by acontrolled system (not shown) which will react in response to a detectederror signal. For this purpose we could use, for example, a dedicatedtrack of the magnetic tape that acts as a synchronization track. Wecould also, for example, monitor the average output signal power of thesensor cells 108 in order to achieve optimal and continuous compensationfor the wear of the read head.

For reasons of visibility, in the FIGURE the flat parallel-faced plates109 and 112 are shown only in their rest positions when the magnetictape is centered and the read head has not yet suffered any wear. Inthis situation, the beams 105 and 106 are those effectively transmittedby the flat parallel-faced plates, whereas the beams 115 and 116 arethose that would be transmitted when these plates have turned on theaxes 113 and 114 through the angles necessary to obtain the requireddeviations. The paths of these beams in the plates, as illustrated inthe FIGURE, are therefore not the real ones; they are shownschematically to enable the invention to be understood.

The invention can be extended to all means enabling an automaticcompensation for wear of the head, such as for example a micromotor thatmoves the head itself—or just its sensitive part—so as to keep thereading beam on the active part of the head.

1. A magneto-optical reading device for multi-track magnetic tapes,comprising: a magnet-optical read head configured to receive a flatlarge incident beam at an active part on an initial surface of themagneto-optical read head configured to contact the multi-track magnetictape prior to any wear on the initial surface due to passage of themulti-track magnetic tape and to reflect the flat large incident beamfrom the active part onto an array of sensors as a reflected beam; and acorrection device configured to move a zone of incidence of said flatlarge incident beam from the active part on the initial surface to a newactive part on a worn surface on said read head in order to keep saidbeam optimally positioned on said active part of said read head afterwear of the initial surface due to the passage of the multi-trackmagnetic tape.
 2. The magneto-optical reading device according to claim1, wherein said correction device comprises a first flat parallel-facedplate positioned on a path of said flat large incident beam, said firstflat parallel-faced plate being configured to be rotatable around afirst axis parallel to a plane of said flat large incident beam andperpendicular to an axis of the flat large incident beam.
 3. Themagneto-optical reading device according to claim 2, wherein thecorrection device further comprises a second flat parallel-faced platepositioned on the path of the reflected beam returned by said read head,said second flat parallel-faced plate being configured to be rotatablearound a second axis parallel to a plane of said reflected beam andperpendicular to an axis of the reflected beam, this second plate beingprovided to correct a displacement of the reflected beam associated witha displacement of said incident beam caused by a rotation of said firstplate.
 4. The magneto-optical reading device according to claim 3,wherein said second plate further configured to have a third rotationaxis perpendicular to the plane of said reflected beam so as to be ableto move the reflected beam in a plane including the reflected beam inorder to keep spot beams of the reflected beam modulated by magnetictape tracks of the multi-track magnetic tape on corresponding sensors ofsaid array of sensor, and thereby compensate for any wandering movementsof the multi-track magnetic tapes.
 5. The magneto-optical reading deviceaccording to claim 1, further comprising a control system configured todrive said correction device to assure permanent and optimal movement ofthe zone of incidence to correct for wear on the initial surface.
 6. Amagneto-optical reading device for multi-track magnetic tapes,comprising: a magnet-optical read head means for receiving a flat largeincident beam at an active part on an initial surface of themagneto-optical read head means configured to contact the multi-trackmagnetic tape prior to any wear on the initial surface due to passage ofthe multi-track magnetic tape and to reflect the flat large incidentbeam from the active part onto an array of sensors as a reflected beam;and a correction means for moving a zone of incidence of said flat largeincident beam from the active part on the initial surface to a newactive part on a worn surface on said read head means in order to keepsaid beam optimally positioned on said active part of said read headmeans after wear of the initial surface due to the passage of themulti-track magnetic tape.
 7. The magneto-optical reading deviceaccording to claim 6, further comprising a control means for drivingsaid correction means to assure permanent and optimal-movement of thezone of incidence to correct or wear on the initial surface.